Reddit has a protection against some common patterns used by upvote bots. There is a possibility to buy some kind of upvote service that then doesn't work effectively and doesn't deliver upvotes.
That's what happened to me with redditmarketingpro and upvotes.io. The sad part is that redditmarketingpro never even refunded my money despite their inability to deliver. upvotes.io on the other hand, somehow got around Reddit's tight security and got it working within an hour.